1. Field of the Invention
The present invention relates to a thermal printing method and, more particularly, to a method of printing without margins along the leading and trailing edges of a sheet of printing paper.
2. Description of the Related Art
Generally, a thermal printer has a transfer structure such that, an ink film ribbon is placed between a thermal print head and printing paper which is the object of the printing operation, so that ink sublimated by the thermal print head to a predetermined pattern is transferred onto the printing paper. In a color thermal printer, analytic images of yellow, magenta and cyan, which have been color-analyzed for full color implementation, are repeatedly printed three times, respectively.
FIG. 1 is a plan view of a sheet of printing paper printed by a conventional printing method. As shown in FIG. 1, an actual image 1 printed on the printing paper is printed such that a margin 2 is present along the edges of the printing paper. Such a conventional printing method will be explained with reference to FIGS. 2, 3 and 4.
First, FIG. 2 is a side view of a drum type printer for explaining the conventional printing method.
Referring to FIG. 2, a printing paper cassette 9 is provided on one side of the lower part of drum 4. A paper supplying roller 7, which is placed at the outer periphery of drum 4, supplies printing paper 3 from the printing paper cassette 9. A clamp 5 for clamping the leading edge of each sheet of the printing paper 3 and for transferring the printing paper to drum 4 is provided on the drum 4. Above drum 4, a thermal print head 10 heated by a heat generating element is provided so as to be capable of moving up and down. A ribbon of ink film 12 is provided between thermal print head 10 and the printing paper 3, so that ink is sublimated by the heat and pressure of the thermal print head to print an image on the printing paper.
In the above conventional thermal printer, first, printing paper 3 is transferred from printing paper cassette 9 to drum 4. Then, if the printing paper 3 is located between the drum 4 and the thermal print head 10, the clamp 5 clamps the leading edge of the printing paper 3. Subsequently, the clamp 5 and the drum 4 are rotated to transfer the leading edge of printing paper 3 to a position offset from the position directly under thermal print head 10. This offset position is selected because, if clamp 5 were located at the initial printing position (a position A' directly under thermal print head), the clamp 5 would interfere with the thermal print head 10 when the print head is lowered for printing.
In contrast to the drum type printer of FIG. 2, FIG. 3 shows a platen type printer according to another conventional printing method. Here, the difference is that, instead of clamp 5 which clamps the leading edge of printing paper 3 to transfer the printing paper to the drum 4, guide rollers 8 and 8' are provided for transferring printing paper 3 from printing paper cassette 9 and guiding the printing paper while in contact with the drum 4.
In the platen type printer shown in FIG. 3 which utilizes guide rollers 8 and 8' since the printing paper tends to separate from drum 4 before it arrives at the initial printing position A' of the thermal print head 10, the thermal print head 10 is not moved down for printing until printing paper 3 is transferred by guide roller 8'. Thus, it is difficult to print on the leading margin of the printing paper 3.
FIG. 4 shows a capstan roller type printer according to still another conventional printing method.
Comparing the printer shown in FIG. 4 with those shown in FIGS. 2 and 3, the printing paper 3 is transferred and guided by the rotation of capstan rollers 8" and 8"' instead of drum 4. Drum 14 of FIG. 4 supports printing paper 3 transferred by capstan rollers 8" and 8"' and, when the thermal print head 10 is moved down, also provides support for ink ribbon 12.
In the conventional capstan roller type printer shown in FIG. 4, since thermal print head 10 is not moved to start printing until the printing paper is transferred by the second capstan roller 8"' for tightening the printing paper, the space from the top point of the second capstan roller 8"' to the initial printing position A' remains as a margin on the leading edge of printing paper 3. At the same time, the space from initial printing position A' to the top point of the first capstan roller 8" remains as a margin on the trailing edge of printing paper 3.
U.S. Pat. No. 5,001,498 discloses a thermal printer which, for eliminating the margin of the printing paper generated in the capstan roller type printer shown in FIG. 4, prints from the leading edge of the printing paper to the trailing edge thereof, by transferring the printing paper until the leading edge of the printing paper arrives at a specific point of a platen roller supporting the ink film and the paper, and then performing a printing operation. In the above patent, printing paper is transferred by the pressing of the roller and prints without leading and trailing margins.
On the other hand, the left and the right margins shown in FIG. 1, can be eliminated by controlling the size of thermal print head 10 (shown in FIGS. 2-4), but the leading margin of the printing paper still remains. Therefore, for the sake of appearance, left, right and trailing edge margins should also be reserved.
This margin does not provide a good appearance and wastes printing paper. Also, when an image input through an image input apparatus such as a scanner is printed by the above printer, the image to be printed is compressed and/or truncated before being printed.